A process for regenerating used foundry sand is known from DE-OS 34 00 656, for example. In this known process the used sand is supplied via a magnetic separator to a rotary drum having mechanical baffles where it is dried by means of hot air passed through the drum. The drum simultaneously serves for comminuting the sand components, if necessary, and mechanically removing the binder components adhering to the sand grains by means of friction. The grinding bodies present in the rotary drum may also serve this purpose. However, this mechanical removal may also be carried out in a separate step by means of an impingement separator as known from DE-PS 28 56 536 or DE-PS 31 10 578, for example.
After temporary storage, the sand is passed through, a fluid-bed furnace in the prior art process, in which namely the chemical components, such as organic binders, are burned off at temperatures of around 800.degree. C. The hot sand is then passed through a cooling drum where the sand is subsequently cleaned and cooled to room temperature by means of cooling air. Then, the sand can be classified and reused. Hot outlet air of the fluid-bed furnace is used to dry the sand, which air is supplied to the drying drum through, an outlet-air filter and, after renewed filtering, is discharged together with all gaseous pollutants into the atmosphere impairing the environment. The outlet air of the cooler and the air used for conveying the sand is also discharged into the atmosphere after passing the filter.
The reprocessing of the used sands in forges has become increasingly important, since a simple disposal of the used sands faces increasing difficulties for ecological reasons. Thus, regeneration plants for used sands are employed increasingly. These used sands may contain chemical additives, particularly organic binders as well as inorganic binders such as clays. The regeneration of used sands having high proportions of clay creates special difficulties. (In the case of natural sands and clay sands) the clay proportion may be present as kaolinite, as montmorillonite (main component of bentonite) and as mullite or aluminosilicate (an important component of chamotte).
A purely mechanical removal or regeneration of the used sands will only be satisfactory if they have a relatively low clay content. As in the process according to the abovementioned OS 34 00 656, frequently a thermal treatment step is introduced in which the used sands are heated to temperatures of about 800.degree. C. At these temperatures, the organic binders are burned and the clay-containing components are baked to a considerable extent as mullite onto the quartz sand grains. The sand is chamotted in this way, the chamotte portion frequently reaching 5% or more in the regenerated sand. In this case, it proved to be disadvantageous that the chamottization results in an increased consumption of certain binder components. In addition, the sand surface becomes considerably porous, which also substantially raises the binder consumption. The combustion of the organic components also adds thereto, since additional cavities form in the quartz grain by this combustion.
Another drawback of heating the sand to elevated temperatures is represented by the high demand for thermal energy. Furthermore, the sand grains have to be subjected to another mechanical removal after this heating. These drawbacks can partially be eliminated successfully when the used sand is regenerated in a wet state. However, this leads to high costs and considerable problems with respect to the disposal of the resulting sewage sludge.